In ink-jet recording methods, ink droplets are generally formed by using any of various ink discharge methods. Such ink-jet recording methods include applying a high voltage to form an electrostatic attraction or applying mechanical vibration or displacement to an ink using a piezoelectric element. Another ink-jet recording method heats an ink to generate bubbles and utilizes the resulting pressure. These methods provide ink droplets and force the ink droplets to fly and subsequently adhere as ink dots to a recording material, such as a recording paper, thereby performing the recording. Ink jet recording methods typically generate less noise and allow high-speed printing or multi-color printing.
Ink-jet recording methods can be used to generate images on various types of recording material, for example plain paper, dedicated recording paper; and optical transparency sheets for a slide or overhead projector (OHP). Other ink jet recording methods utilize a color-separation plate for forming a positive plate for color printing; or a color mosaic filter (CMF) for a color display device such as a liquid crystal device; and the like.
When a color image is formed by the ink-jet recording method, various colors are generally developed using a subtractive color mixing method by overprinting inks of three primary colors (yellow (Y), magenta (M) and cyan (C)), on one another in an appropriate combination on the same pixel. For example, red (R), green (G), blue (B) and black (BK), can be generated by mixing the combinations of primary colors as shown in the following expressions:Y+M→RY+C→GM+C→BY+M+C→BK
Red (R) can be developed by appropriately mixing yellow (Y) and magenta (M), green (G) can be developed by appropriately mixing yellow (Y) and cyan (C), blue (B) can be developed by appropriately mixing magenta (M) and cyan (C), and black (BK) can be developed by appropriately mixing yellow (Y), magenta (M) and cyan (C). However, when a black image is formed by overprinting inks of three primary colors (yellow, magenta and cyan), on one another, the volume of the ink droplets on a recording material becomes large. The line becomes thicker compared with portions where colors other than black are recorded resulting in an unnatural image, and ink absorption failure of the recording material occurs resulting in lowering of the drying property and the like.
In many cases, text and lines are formed in black, thus high optical density, sharpness and lightfastness are required for the black text and black lines. Often, therefore, a black ink is added to an ink set for ink-jet recording in addition to color inks of yellow, magenta and cyan.
A black dye or a black pigment such as carbon black often is used as a coloring agent for black ink. However, a black ink containing carbon black as a coloring agent has a problem in that the fixing property of the ink is poor, especially when printing on a glossy paper or on a glossy texture.
Since black ink has a high optical density per se, when a black image is formed only with a black ink, it is often difficult to develop an image with gradation in an area of a medium lightness to a high lightness. It is particularly difficult to reduce the graininess in a black image in an area of a high lightness.
Generally, “a black image of a medium lightness to a high lightness” means an image in dark gray to light gray and specifically, an area with a lightness (L*) of about 30 or more and about 80 or less and with a chroma (C*) of about 50 or less.
In response to the problem of the image quality of such a black image of a medium lightness to a high lightness, a method using plural black inks having different densities as black inks has been proposed (U.S. Pat. No. 6,596,065). However, the use of plural black inks having different densities increases the number of inks constituting an ink set, resulting in increased cost and increased size of an ink-jet recording apparatus.